Ion implantation is a standard technique for introducing property-altering impurities into substrates. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the substrate. The energetic ions in the beam penetrate into the sub-surface of the substrate material and are embedded into the crystalline lattice to form a region of desired conductivity or material property. Depending on the type of device being fabricated using ion implantation, the impurity ions may need to be implanted at various depths within the crystalline lattice. The depth of implantation is determined by the energy of the ions directed toward the substrate. For example, certain devices require shallow transistor junctions necessitating low energy ion beams for implantation. Conversely, certain devices fabricated for use in power applications, for example, require deeper impurity implantation necessitating a high energy hydrogen ion beam.
For these high energy implantation applications, typically 750 kV or greater, tandem acceleration is often used to generate ions having the required energy. Often tandem acceleration is applied to hydrogen ions in order to generate high energy ions sufficient for deep implants into a substrate. In a typical tandem acceleration process, an electrostatic accelerator accelerates negative hydrogen ions generated in an ion source from ground potential up to a positive high-voltage terminal. The electrons on the negative hydrogen ions are then stripped from the negative ion by passage through a charge exchange region, the stripper. The resulting positive hydrogen ion (proton) is again accelerated as it passes to ground potential from the high positive potential. The protons emerge from the tandem accelerator with twice the energy of the high positive voltage applied to the tandem accelerator. In order to maximize transmission of the accelerated ion beam through the tandem accelerator, the ion beam must be transported through the stripper which is located at the nominal center of the tandem accelerator. An over-focused beam occurs when the beam converges at its focal point at a position along the beam path that does not coincide with the location of the stripper which is the nominal center of the tandem accelerator, namely that the beam is focused before the nominal center of the tandem accelerator. An over-focused beam yields deleterious effects on beam transmission and associated beam current. Accordingly, there is a need for an improved high energy ion implantation system which prevents the transmission loss of the over-focused ion beams supplied to a tandem accelerator to increase beam current. It is with respect to these and other considerations that the present improvements have been needed.